A lens system of a zoom lens device used for a surveillance video camera typically includes a focusing lens group and a variator lens group (Patent Document 1 and Patent Document 2). Lens systems having a four-group configuration including a focusing lens group, a variator lens group, a compensator lens group, and a master lens group arranged in this order from the object side are actually used widely. Since near infrared light is used for the illumination for surveillance at night, fluoric lenses or ED lenses are used for these lens systems in order to minimize aberration between visible light and near infrared light.
Some zoom lens devices used in surveillance video cameras include a preset mechanism. The positions of the focusing lens group and variator lens group for taking an image of a predetermined location to be imaged with a predetermined magnification are determined in advance, and the focusing lens group and variator lens group are adjusted to these positions predetermined by the preset mechanism when an image of that location is actually taken. To prevent defocusing caused by an ambient temperature change in a zoom lens device equipped with such a preset mechanism, Japanese Patent Application Laid-Open No. Hei 11-205655 proposes providing a temperature sensor to the zoom lens device and separately correcting the positions of the focusing lens group and variator lens group on the basis of an initially set temperature and a current temperature.